1. Field of the Invention
The present invention relates to a sheet-fed rotary printing press.
2. Description of the Prior Art
Sheet-fed rotary printing presses are classified into a lithographic press, a letterpress machine, and an intaglio printing press in accordance with the types of plates used. Of these printing presses, a multicolor lithographic press is arranged as shown in a schematic side view of FIG. 4. In FIG. 4, a printing press 1 consists of a paper sheet feeder 2, a plurality of printing units 3, and a delivery apparatus 4. The paper sheet feeder 2 consists of a stack board 6 which is automatically moved vertically while supporting paper sheets 5 thereon, and a paper pick-up device (not shown), arranged above the sheets 5, for drawing the paper sheets 5 by suction one by one from the uppermost sheet and feeding the drawn sheets onto a feedboard 7. Each printing unit 3 consists of a plate cylinder 8, a blanket cylinder 9, and an impression cylinder 10, which are in contact with each other. A transfer cylinder 11 and a transfer cylinder 12 having a diameter twice that of the transfer cylinder 11 are disposed between the impression cylinders 10 of the adjacent printing units 3, so that their peripheral surfaces are in contact with each other. Swing grippers 13 are arranged obliquely above the impression cylinder 10 of the first color, and a delivery cylinder 14 is in contact with the last impression cylinder 10. Reference numerals 15 denote a right and left sprocket which is coaxially supported on each end of the delivery cylinder 14. A right and left delivery chain 18 is looped respectively between the sprockets 15 and a right and left sprocket 17 disposed at the front end portions of a delivery frame 16 so as to be adjustable by adjustable right and left sprockets 19. A plurality of delivery grippers (not shown ) are arranged in parallel on a gripper rod (not shown ) suspended between the right and left delivery chains 18 at equal intervals. A stack board 20 which is automatically moved vertically while supporting the sheets 5 thereon is arranged below a rear end portion of a conveyance path of the delivery chains 18. These devices are driven from the side of the transfer cylinder 12 for the first color by a motor 21.
With the above arrangement, the sheets 5 on the stack board 6 drawn by suction by the paper pick-up device (not shown) are gripped one by one by the swing grippers 13 and are then gripped by grippers of the impression cylinder 10. When the sheet 5 which is conveyed while being alternately gripped by the grippers of the impression cylinders 10 and the transfer cylinders 11 and 12 passes between the blanket cylinder 9 and the impression cylinder 10 for each color, an image transferred on the blanket surface of the blanket cylinder 9 from a plate surface is transferred onto the sheet 5, thus performing printing. The sheet 5 on which an image of the final color is printed is regripped from the impression cylinder 10 to the delivery grippers of the delivery chains 18 upon cooperation of the delivery cylinder 14, and is then conveyed by the delivery chains 18 which travel in a direction indicated by an arrow in FIG. 4. Thereafter, the sheet 5 is released from the grippers at the rear end portion of the conveyance path, and is stacked on the stack board 20.
In the printing operation described above, misregistration, contamination with oil, offset, and the like occur due to various causes. An abnormal sheet must be removed so as not to be mixed in and stacked together with the normal sheets.
For this purpose, an abnormal sheet detecting apparatus has been recently developed. In this apparatus, pixels from the leading edge to the trailing edge of sequentially conveyed printed products are detected using a sensor or a camera, and a difference between the detected density and a density of pixels of a reference image is discriminated, so that an abnormal sheet is delivered onto a separate delivery tray or a mark is printed on the abnormal sheet in accordance with the discrimination signal. Some users want to add the detecting apparatus to their printing press.
However, in order to accurately operate the abnormal sheet detecting apparatus, an apparatus for tensioning a printed product as an object to be detected during detection must be provided, and the abnormal sheet detecting apparatus must be provided on the downstream side of the paper path of the final printing unit. Therefore, if this apparatus is provided to the existing printing press, the printing press must be greatly modified. However, since most of printing presses cannot be modified, the above detecting apparatus has not yet been used in practical applications although it is in demand.